1. Field of the Invention
The present invention relates to a phase locked loop (PLL), and more particularly to a PLL without a charge pump and an integrated circuit having the PLL.
2. Description of the Related Art
Various electronic devices include a PLL for stabilizing a generated signal having a desired frequency.
Generally, a PLL can include a phase-frequency detector, a charge pump, a loop filter, a voltage-controlled oscillator (VCO) and a divider. The PLL detects a phase/frequency difference between an input signal (FIN) and a divided signal (FOUT/M) and controls a phase/frequency of an output signal (FOUT) based on a result of the detected phase/frequency difference.
The VCO performs based on a control voltage outputted from the loop filter. For example, when the control voltage is relatively low, the VCO can output an output signal (FOUT) having a relatively low frequency, and when the control voltage is relatively high, the VCO can output an output signal (FOUT) having a relatively high frequency.
FIG. 1 is a graph illustrating relations between a frequency of an output signal and a control voltage.
Referring to FIG. 1, a slope of a first line 110 is steeper than a slope of a second line 120. Thus, a frequency change of an output signal (FOUT) according to the control voltage in the first line 110 is greater than a frequency change of an output signal (FOUT) according to the control voltage in the second line 120. As such, a VCO need to operate based on a control voltage with a wide input range so as to obtain an output signal (FOUT) having a stable frequency.
A conventional PLL is disclosed in “L. Sun and D. Nelson, A 1.0V GHz Range 0.13 μm CMOS Frequency Synthesizer, IEEE Custom Integrated Circuits Conference, pp. 327-330, May 2001” and “R. Gu, et al., A 6.25 GHz 1V LC-PLL in 0.13 μm CMOS, ISSCC Digest of Technical Papers, pp. 594-595, February 2006”, in which a charge pump is improved to obtain a control voltage with a wide input range so as to generate an output signal (FOUT) having a stable frequency. However, the charge pump in the conventional phase loop is complex, and the charge pump is required to be performed by a low voltage in a submicron process.